exogear, I ****ing love it, pick it up at best buy or any electronics store for about 60 bucks, better than the mophie, it's the slimmest batter pack they have out for the iPhone, plus it fits in your hand sooooo nice, perfect fit. EXOGEAR exolife for iPhone 4

I have the Griffin Powerduo Reserve. Small battery fits in your pocket and works great. It boosted my battery from 57% to 86% at work last night. I highly recommend it. It comes with a car charger and one for the home. It's awesome.